Nerve Impulses

Sec 17.1Pg 324-325

Memory Makers from Last Class…

Fingerlike Extensions

Outskirts and Organs Please PNS

Conductor Tubes

Go between

Motors Away, Make Move!

Fried Egg

Brain is the Centre

Feeling it to the CNS

Can you guess which vocab word from last class goes with each group of words?

Memory Makers side 2

Leap Frog

Negative Nancy NoPulse

Support Worker

Report Positive Change

Extension Cord

Lights Camera Impulse aka Action

All or nothing

Against the FLow

Huge rorqual whales have nerves like 'bungee cords,' UBC scientists learn

Did you know..Squids have giant axons which are

used to study nerve impulses!

The Nerve Impulse The way a neuron transmits

information!

A wave of changes in charge (polarity) that travel from the dendrites to the axon

Voltage changes measured using a voltmeter and on an oscilloscope

The Nerve Impulse A nerve impulse is composed of three stages:

1. Resting potential.

2. Action Potential.

3. Recovery Period.

What is happening inside the axon?

Let’s take a look!

Your Job Today: Follow along with your notes

package…

Highlight the points emphasized in our discussions…

Complete the summary table

After class, go back and draw in the ions/charges in the axons & REVIEW!!!

Resting Potential In order to understand a nerve impulse, it is

important to understand the nature of the axon when it is at rest (not stimulated).

There are three things which determine the axon’s resting potential:

1. Sodium ions (Na+)

2. Potassium ions (K+)

3. large, negatively charged proteins

Resting Potential There are some other structures which also

play important roles:

1. SODIUM-POTASSIUM PUMPS

(intrinsic carrier proteins in the cell membrane of the axon) which actively transport:

Na+ ions OUT OF the neuron

K+ ions INTO the neuron

Resting Potential2. CHANNEL PROTEINS

(intrinsic proteins in the cell membrane of the axon) means the membrane is somewhat permeable to the ions which diffuse down their concentration gradients:

Na+ ions diffuse back INTO the neuron

K+ ions diffuse OUT OF the neuron

The membrane is more permeable to potassium than sodium, so there are more positively charged ions outside the membrane.

Resting Potential3. LARGE, NEGATIVELY CHARGED PROTEINS

located within the neuron and are too large to move out.

Resting Potential The net result of:

• the sodium-potassium pump

• the greater leakage of potassium ions

• the large, negative proteins in the cell

IS

an overall NEGATIVE CHARGE on the inside the membrane

an overall POSITIVE CHARGE outside the membrane.

The overall charge is about -65 millivolts (mV) inside compared to outside.

Resting Potential

Action Potential A stimulus is any factor which causes a

nerve impulse to be generated.

Examples are an electric shock, a change in temperature, physical pressure on a cell and so on.

A stimulus will disrupt the resting potential by disrupting the distribution of Na+ and K+ ions.

Action Potential A stimulation of the neuron will cause:

The temporary opening of SODIUM GATES which makes the membrane suddenly more permeable to Na+ ions.

Na+ ions diffuse INTO the axon due to the concentration gradient (high [Na+] outside membrane, low [N+] inside membrane).

This causes the charge difference to change from -65 mV to +40 mV. This reverse in polarity is called DEPOLARIZATION.

Depolarization

Action Potential The immediate response (within a half a

millisecond) to the change in polarity is another change in the permeability of the neuron’s membrane.

POTASSIUM GATES now open which means K+ ions diffuse OUT of the neuron.

Sodium gates have closed.

This causes the charge difference to change from +40 mV to -65 mV again.

This reverse in polarity is known as REPOLARIZATION.

Note that although the initial polarity has been restored, the Na+ and K+ ions are in the reverse positions.

Repolarization

Action Potential The DEPOLARIZATION and REPOLARIZATION

of the neuron is referred to as the ACTION POTENTIAL.

Recovery Period During the recovery period, the sodium-

potassium pumps work to return the ions to their original concentrations.

Na+ is moved out of the neuron and K+ is moved in.

During the recovery period, the neuron is unable to conduct a nerve impulse. (See diagram for resting potential.)

*In your text this is referred to as the REFRACTORY PERIOD.

Action Potential Animations

http://psych.hanover.edu/Krantz/neural/charge2.html

http://www.blackwellpublishing.com/matthews/channel.html

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#

Action Potential Summary

Resting Potential:

charge is -65 mV

Na+ is outside neuron

K+ is inside neuron

Action Potential (depolarization):

charge moves from -65 to +40 mV

Na+ gates open & Na+ moves inside axon

K+ also trapped inside axon

Action Potential Summary

Action Potential (repolarization):

charge moves from +40 to -65 mV

K+ gates open AND K+ moves outside

Na+ gates close, Na+ stays inside

Recovery Period:

charge is -65 mV (back to resting potential)

Na+/K+ pump moves against gradient Na+ out K+ in

Threshold: All or None Response

In order for an action potential to be initiated, the stimulus must be above a minimum level.

The minimum level required to activate an action potential in a neuron is called the THRESHOLD.

It is usually 10 to 20 millivolts above resting potential.

Any stimulus less than the threshold will not produce an impulse.

Any stimulus greater than the threshold will produce an impulse.

Threshold: All or None Response

The strength of a nerve impulse is always the same as long as the threshold is reached.

This is why a nerve impulse is called an all or none response.

Either there is an impulse or there is not.

There can be no “partial” nerve impulse.

A stronger stimulus does not result in a bigger impulse, rather it means a greater number of impulses (more nerves involved or a single nerve conducting a series of impulses).

Oscilliscope Reading of an Impulse

Saltatory Conduction Most axons are covered by tightly packed

spirals of SCHWANN CELLS.

These encircle the axon and lay down layers of cellular membrane.

This cellular membrane contains a fatty substance called MYELIN which acts as an electrical insulator.

Saltatory Conduction Between the sheaths of myelin (Schwann

cells) are gaps called NODES OF RANVIER.

As an impulse travels down a myelinated axon, it can jump from node to node rather than traveling the full length of the axon.

This greatly increases the speed of the nerve impulse and is known as SALTATORY CONDUCTION

Animations http://www.blackwellpublishing.com/matthew

s/actionp.html

http://www.brainviews.com/abFiles/AniSalt.htm